The subject matter disclosed herein relates generally to diagnostic imaging systems, and more particularly to positron emission tomography (PET) diagnostic imaging systems and correction of emission scatter.
PET imaging systems typically generate images depicting the distribution of positron-emitting nuclides in patients. The positron interacts with an electron in the body of the patient by annihilation, with the electron-positron pair converted into two photons. The photons are emitted in opposite directions along a line of response. The annihilation photons are detected by detectors (that are typically in a detector ring assembly) on both sides of the line of response on the detector ring assembly. These detections are termed coincidence events. The coincidence events detected by the PET detector ring assembly are typically stored within data structures called emission sinograms, which is a histogram of the detected coincidence events. An image of the activity distribution within a patient's body is generated from the emission sinograms through a process called image reconstruction.
Some photons are deflected from their original direction and such events are termed scatter events or scattered coincidences. It is desirable to reject the scatter events during the acquisition of emission sinograms, because images generated using only the detected true coincidence events represent a true activity distribution of radio-activity in the scanned body part of the patient. The deflected photons have less energy than the undeflected photons that comprise true coincidence events, and therefore scatter events can be rejected if one or both detected photons have measured energy substantially less than the 511 keV characteristic of undeflected photons. However, the energy measurement process for the detected photons is not perfect and some scatter events are incorrectly accepted as true coincidence events. The image reconstruction process must account for these detected scatter events in order for the PET imaging system to produce unbiased estimates of the activity distribution in the patient.
In conventional PET imaging systems, such as PET/computed tomography (PET/CT) imaging systems, three-dimensional (3D) PET model based scatter estimates are used to correct for the effects of photon scatter. The model based algorithms of these conventional systems estimate the shape of a scattered photon distribution in PET data. The shape is then adjusted to an accurate scaling level by comparing the magnitude of the model based estimate and the measured PET data outside the patient's body. For a PET scan, this method may be used because all of the sources are typically inside the body. In a PET/CT system, CT images or sinogram data can be used to estimate the boundary of the patient.
These methods, however, are sensitive to misregistration between the CT and PET data. Accordingly, if a patient moves, for example, moves a hand, arm or head, between the CT and PET portions of the PET/CT exam, valid photon count data may be interpreted as scatter because the conventional methods do not necessarily provide a proper scaled version of the scatter estimate. That is, a region of the PET sinogram that is assumed to have only scattered events, because the region is outside of the patient as determined by the CT imaging, also includes true PET emission events. These true emission events are then used to scale the scatter estimate, thus resulting in overscaling, and accordingly an overestimate of scatter.